Recently, as natural resource issues and global warming issues have become ever more serious, much research and development efforts have been invested in reducing energy consumption of heat pump apparatus used for water heaters and air conditioners. For example, conventional heat pump apparatuses have a mechanism of expanding refrigerant using an expansion valve, but there is an attempt to recover the energy of expansion of the refrigerant by employing a positive displacement expander to utilize it as auxiliary power for the compressor. Theoretically, through the recovery and utilization of the expansion energy of the refrigerant, about 20% reduction in power usage can be expected, or even with an actual apparatus about 10% reduction in power usage can be expected. As a fluid machine that realizes such an attempt, development of an expander-compressor unit, such as disclosed in JP 2005-299632 A, is underway at a rapid pace.
FIG. 17 is a vertical cross-sectional view illustrating a typical expander-compressor unit. An expander-compressor unit 200 is provided with a two-stage rotary type compression mechanism 121, a motor 122, a two-stage rotary type expansion mechanism 123, and a closed casing 120 that accommodates them. The compression mechanism 121, the motor 122, and the expansion mechanism 123 are coupled to each other by a shaft 124.
A bottom part of the closed casing 120 forms an oil reservoir 125 for holding oil (refrigeration oil). An oil pump 126 is attached to a lower end portion of the shaft 124 in order to pump up the oil stored in the oil reservoir 125. The oil pumped up by the oil pump 126 is supplied to the compression mechanism 121 and the expansion mechanism 123 via an oil supply passage 127 formed in the shaft 124. Thereby, lubrication and sealing are ensured in the sliding parts of the compression mechanism 121 and the expansion mechanism 123.
An oil return pipe 128 is disposed at an upper part of the expansion mechanism 123. One end of the oil return pipe 128 communicates with the oil supply passage 127 formed in the shaft 124, while the other end opens downward of the expansion mechanism 123. Generally, excess oil is supplied for ensuring the reliability of the expansion mechanism 123. The excess oil is returned via the oil return pipe 128 to the oil reservoir 125.
The expander-compressor unit has the advantage that the compression mechanism and the expansion mechanism can share the same oil easily since the compression mechanism and the expansion mechanism are disposed in a common closed casing.
On the other hand, there is another attempt in which the expansion force of the refrigerant is not directly transferred to the compression mechanism but is used to perform electric power generation, and the generated electric power is input to the motor. According to this attempt, it is unnecessary to integrate the compression mechanism and the expansion mechanism, so the compression mechanism and the expansion mechanism may be accommodated in separate casings. Although the compression mechanism and the expansion mechanism may be accommodated in separate casings, it is necessary to bear in mind that the oil mixed with the refrigerant circulates in the refrigerant circuit. In other words, some kind of design scheme for balancing the amounts of the oil between the casings is necessary to prevent the amounts of the oil from becoming uneven between the casings so that lubrication deficiency does not occur. On the other hand, such a design scheme is essentially unnecessary for the expander-compressor unit in which the compression mechanism and the expansion mechanism are disposed in a common closed casing.